Harvard's Emergency website issued has cleared the university's Science Center, the last of four sites that were searched for possible explosives.

After nearly 6 hours of searching by bomb squads and other experts, authorities at Harvard University on Monday had issued "all clear" notices for the four buildings apparently cited in "unconfirmed reports" about explosives on campus.

Those reports had led school officials to evacuate the buildings.

But shortly before 1:50 p.m. ET, the school posted word on its website that Sever Hall was "cleared and students may return. Science Center still closed." Students were also being allowed back on to Harvard Yard.

About an hour earlier, officials had announced that "Thayer and Emerson Hall are cleared and students may return."

So, the situation was mostly resolved — though there was still no word on who reported the threat.

The word of a partial "all clear" followed earlier updates from the university. One at 11:48 a.m ET said:

"Report of bombs on campus remains unconfirmed. Federal and state officials have joined the probe. Updates to be posted here."

That was the same message as the previous note, posted at 10:40 a.m. ET:

"As you learned from the MessageMe alerts this morning, the Harvard University Police Department (HUPD) received an unconfirmed report that explosives may have been placed in four buildings on campus: the Science Center, and Thayer, Sever and Emerson halls. Out of an abundance of caution, the buildings have been evacuated while the report is investigated by the HUPD and the Cambridge Police Department.

"Access to Harvard Yard has been restricted to residents of the Yard with Harvard ID. As of the writing of this message the report remains unconfirmed and the HUPD has no reason to believe there is a threat to any other site on campus.

"Harvard's focus is on the safety of our students, faculty and staff. We will continue to update the community via MessageME and through messages posted here and on the University's Twitter and Facebook accounts."

"Alert: Reports of bombs placed on campus are unconfirmed. There have been NO reports of explosions. View for updates: http://hvrd.me/rNXaJ"

Then at 9:33 a.m. it followed with word that officials were acting "out of an abundance of caution":

"The Harvard University Police Department (HUPD) this morning received an unconfirmed report that explosives may have been placed in four buildings on campus: the Science Center, Thayer, Sever and Emerson Halls. HUPD and Cambridge Police are on the scene. Out of an abundance of caution, the buildings have been evacuated while the report is investigated. Harvard's focus is on the safety of our students, faculty and staff. We will update the media when we have more information."

The university is in session this week. Its winter recession officially begins on Saturday. The student-run Harvard Crimsontweets that "final exams for this morning have been canceled." There was applause from some students after that announcement, the Crimson adds.